The uses of polymeric materials, including diolefin polymers, continue to grow rapidly in such diverse areas as protective paint coverings, wire insulation, structural panels for automobiles, piping and lubricating oil viscosity index improvers. In many of these applications, the stability of the polymer is of paramount importance. Hydrogenation of diolefin polymers greatly improves the stability of these polymers against oxidative, thermal, and ultraviolet degradation. Polymer hydrogenation processes have therefore been studied for many years as a method to prepare novel materials with excellent stability and other desirable properties. Early processes utilized heterogeneous catalysts which were known to be useful for hydrogenation of low molecular weight olefins and aromatics. Nickel on kieselguhr is an example of this type of catalyst. A fine catalyst powder was preferred and large amounts of catalyst were required to complete the hydrogenation in a reasonable time. Such processes were only partially successful, since the reaction requires the diffusion of the polymer molecules into the pores of the catalyst, where the active nickel metal is present. This is a slow process when hydrogenating polymers.
Nickel octoate/triethyl aluminum hydrogenation catalyst systems were then developed which enabled rapid hydrogenation of polymers. The catalyst is utilized as a colloidal suspension in polymer containing solutions. This type of catalyst is referred to as a homogeneous catalyst. Such a process has been used for a number of years to prepare hydrogenated butadiene-styrene polymers that are used as viscosity index improvers in premium motor oils. U.S. Pat. No. 3,554,991 describes an exemplary process. Besides nickel, Group VIII metals in general will function as the active metal in these systems, and in particular, iron, cobalt, and palladium are known to be acceptable.
Pore diffusion is not a limitation with homogeneous catalysts and the hydrogenation process is rapid and complete in minutes. However, removal of the catalyst from the polymer product is necessary because metals, and particularly nickel, which remain with the polymer catalyze degradation of the polymer product. The catalyst is typically removed from the polymer solution by the addition of an ammonium phosphate-water solution and air to oxidize the nickel to a divalent state. The mixed nickel-aluminum phosphate can then be removed from the hydrogenated polymer solution by filtration.
Alternative methods to remove hydrogenation catalyst residues from solutions of hydrogenated polymers include treatment with dicarboxylic acid and an oxidant, as disclosed in U.S. Pat. No. 4,595,749; treatment with an amine compound wherein the amine is either a chloride salt or a diamine having an alkyl group of 1 to 12 carbon atoms as disclosed by U.S. Pat. No. 4,098,991; and treatment of the polymer solution with a non-aqueous acid followed by neutralization with an anhydrous base and filtration, as disclosed by U.S. Pat. No. 4,028,485. These processes involve contacting the polymer solution with compounds which contaminate the polymer. Further process steps can be required to remove these contaminants. U.S. Pat. Nos. 4,278,506 and 4,471,099 describe processes to remove such contaminants from hydrogenated polymer solutions. Some of these catalyst removal systems are undesirable because those processes require relatively expensive metallurgy due to the corrosive nature of the compounds. Many also require the consumption of a continuous stream of reactants, and produce an acid sludge containing the catalyst and residues of the treatment chemicals.
U.S. Pat. Nos. 3,531,448; 3,793,306; and 3,793,307 disclose, as comparative examples, processes for removing nickel hydrogenation catalyst residues from polymer solutions by water extraction. In each of these comparative examples fine particles of solids were formed which rapidly plugged a filter when filtration of the solids from the solution was attempted.
It is therefore an object of this invention to provide a process to remove hydrogenation catalyst residue from polymer solutions. It is a further object of this invention to provide a process to remove hydrogenation catalyst residue from polymer solutions which does not require the treatment of the polymer solution with amine, phosphate or sulfate compounds. In another aspect, it is an object of this invention to provide a process which is capable of removing hydrogenation Group VIII metal catalyst residue from polymer solutions to a level of 10 ppm or less based on the polymer.